The aerosol plan is also cheap—so cheap that it completely overturns conventional analysis of how to mitigate climate change. Thomas C. Schelling, who won the 2005 Nobel Prize in economics, has pointed out how difficult it is to get vast international agreements—such as the Kyoto Protocol—to stick. But a geo-engineering strategy like sulfur aerosol “changes everything,” he says. Suddenly, instead of a situation where any one country can foil efforts to curb global warming, any one country can curb global warming all on its own. Pumping sulfur into the atmosphere is a lot easier than trying to orchestrate the actions of 200 countries—or, for that matter, 7 billion individuals—each of whom has strong incentives to cheat.
But, as with nearly every geo-engineering plan, there are substantial drawbacks to the gas-the-planet strategy. Opponents say it might produce acid rain and decimate plant and fish life. Perhaps more disturbing, it’s likely to trigger radical shifts in the climate that would hit the globe unevenly. “Plausibly, 6 billion people would benefit and 1 billion would be hurt,” says Martin Bunzl, a Rutgers climate-change policy expert. The billion negatively affected would include many in Africa, who would, perversely, live in a climate even hotter and drier than before. In India, rainfall levels might severely decline; the monsoons rely on temperature differences between the Asian landmass and the ocean, and sulfur aerosols could diminish those differences substantially.
Worst of all is what Raymond Pierrehumbert, a geophysicist at the University of Chicago, calls the “Sword of Damocles” scenario. In Greek legend, Dionysius II, the ruler of Syracuse, used a single hair to suspend a sword over Damocles’ head, ostensibly to show him how precarious the life of a powerful ruler can be. According to Pierrehumbert, sulfur aerosols would cool the planet, but we’d risk calamity the moment we stopped pumping: the aerosols would rain down and years’ worth of accumulated carbon would make temperatures surge. Everything would be fine, in other words, until the hair snapped, and then the world would experience the full force of postponed warming in just a couple of catastrophic years. Pierrehumbert imagines another possibility in which sun-blocking technology works but has unforeseen consequences, such as rapid ozone destruction. If a future generation discovered that a geo-engineering program had such a disastrous side effect, it couldn’t easily shut things down. He notes that sulfur-aerosol injection, like many geo-engineering ideas, would be easy to implement. But if it failed, he says, it would fail horribly. “It’s scary because it actually could be done,” he says. “And it’s like taking aspirin for cancer.”
I n 1977, the physicist Freeman Dyson published the first of a series of articles about how plants affect the planet’s carbon-dioxide concentrations. Every summer, plants absorb about a tenth of the carbon dioxide in the atmosphere. In the fall, when they stop growing or shed their leaves, they release most of it back into the air. Dyson proposed creating forests of “carbon-eating trees,” engineered to suck carbon more ravenously from the air, and to keep it tied up in thick roots that would decay into topsoil, trapping the carbon. He now estimates that by annually increasing topsoil by just a tenth of an inch over land that supports vegetation, we could offset all human carbon emissions.
Dyson’s early geo-engineering vision addressed a central, and still daunting, problem: neither sulfur-aerosol injection nor an armada of cloud whiteners nor an array of space-shades would do much to reduce carbon-dioxide levels. As long as carbon emissions remain constant, the atmosphere will fill with more and more greenhouse gases. Blocking the sun does nothing to stop the buildup. It is not even like fighting obesity with liposuction: it’s like fighting obesity with a corset, and a diet of lard and doughnuts. Should the corset ever come off, the flab would burst out as if the corset had never been there at all. For this reason, nearly every climate scientist who spoke with me unhesitatingly advocated cutting carbon emissions over geo-engineering.
But past international efforts to reduce emissions offer little cause for optimism, and time may be quickly running out. That’s why a few scientists are following Dyson’s lead and attacking global warming at its source. David Keith, an energy-technology expert at the University of Calgary, hopes to capture carbon from the air. He proposes erecting vented building-size structures that contain grids coated with a chemical solution. As air flows through the vents, the solution would bind to the carbon-dioxide molecules and trap them. Capturing carbon in these structures, which might resemble industrial cooling towers, would allow us to manage emissions cheaply from central sites, rather than from the dispersed places from which they were emitted, such as cars, planes, and home furnaces. The grids would have to be scrubbed chemically to separate the carbon. If chemists could engineer ways to wash the carbon out that didn’t require too much energy, Keith imagines that these structures could effectively make our carbon-spewing conveniences carbon-neutral.
The question then becomes where to put all that carbon once it’s captured. Keith has investigated one elegant solution: put it back underground, where much of it originated as oil. The technology for stashing carbon beneath the earth already exists, and is routinely exploited by oil-well drillers. When oil wells stop producing in large quantities, drillers inject carbon dioxide into the ground to push out the last drops. If they inject it into the right kind of geological structure, and deep enough below the surface, it stays there.